In many security systems there is a general requirement to be able to monitor the position and/or status of one or more surveillance targets or objects. In the marine security application which will be particularly described in more detail hereinafter, the surveillance targets or "objects" may be the crew members on board a yacht, with the object of surveillance being to monitor that all crew members are safely on board, responding to a crew loss event by generating a "man overboard" signal and initiating the operation of sophisticated survival and retrieval equipment.
In other applications the surveillance "objects" may be animate or inanimate and the nature of the monitored event may be one of a number of different possibilities depending on the particular circumstances. For example, if the "object" under surveillance is a case carrying cash or valuables, the "event" may be release of the carrying handle by an authorised operator. This event may be perfectly normal, for example during the everyday handling of the case, placing it on a counter or in a motor vehicle for transport, but may be an alarm "event" in that the handle may only be released by the operator because it has been forced from his grasp by thieves. In order to distinguish between "normal" and "alarm" events the system of the present invention incorporates position monitoring or surveillance equipment operable to trigger appropriate alarm equipment when an alarm event is detected.
In such surveillance monitoring situations there is an essential requirement to conserve the power of an electrical supply since this is usually very limited and required to remain active over an extended period of time. For example, on board a yacht there is only a very limited supply of electricity, either from a small generator or from storage batteries, and opportunities for re-charging the batteries are often severely limited by the weather. For this reason electrical systems avoiding a constant current drain at least in some of their parts have considerable advantages.
In the above indicated application of a security system for monitoring the crew on a boat one physical phenomenon which is available for detection to trigger a "man overboard" indication would be immersion in water since this is an inevitable corollary to falling overboard. However, the crew of a boat, particularly a sailing yacht, are frequently entirely saturated even when performing their normal duties on board in inclement weather and it would be counter productive if the saturation of any sensor carried by the crew caused spurious alarm indication. Indeed, there is a risk that this may result in the crew inhibiting the operation of the alarm sensors in just those conditions in which they are most likely to be required. For this reason the specific embodiment of the security system of the present invention described hereinafter incorporates position discrimination means in combination with a water immersion sensor to produce an output alarm indication only upon coincidence of the water-triggered alarm sensor and detection of the signal from a position remote from the vessel.